1. Field of the Invention
This invention pertains in general to computer network performance and in particular to the measurement of computer network latency.
2. Description of the Related Art
One useful way to measure the performance of a network linking multiple computers is to measure the round trip time (RTT) for a packet to travel from a source network node to a destination network node and back again over a network path. The RTT for a packet is the sum of two components: the time the packet actually spends traveling between nodes (referred to as the base round trip time or “bRTT”), and the time the packet spends queued waiting to be sent from a network node along the network path (referred to as the queue delay, or “Q”). The bRTT can be expressed mathematically as a relationship between a measured RTT (“mRTT”) and the queue delay Q:bRTT=mRTT−Q  (1)The bRTT is a fixed quantity. The queue delay Q for a packet, however, may vary depending upon the network traffic at each node along the network path.
When measuring the RTT, it is useful to separate out the bRTT from the queue delay in order to get a more accurate measure of how long it would take for a packet to travel a given network distance under best case circumstances. However, existing tools for calculating bRTT use bandwidth-intensive techniques that are adequate for small to medium scale use, but are not practical for large-scale use. For example, the Ping tool measures the RTT of a packet from source node to a destination node but does not distinguish between time the packets spend traveling and the time the packets spend in queue delay. The mRTT provided by Ping thus provide only an upper bound on the bRTT between the source and destination nodes. Another tool, Traceroute, measures RTT between each node on a network path, but still does not distinguish between time spent queued at a node, and time spent traveling between a node.
Ping and Traceroute can be run multiple times on the same network path, and the lowest mRTT can be used as an estimate of the bRTT for the path. But this technique consumes bandwidth for the multiple network packets and is cost-prohibitive if the network performance is being measured in real time, or if multiple network paths are being measured.